This invention relates to computerized tomography and, more particularly, to monitoring for the existence and location of a faulty detector in a detector array of a computerized tomography system.
Computerized tomography (CT), or "computed tomography," is an imaging technology in which an array of detectors generate data from energetic rays transmitted through or emitted from an imaging object. For example, a transmission-type medical CT imaging system uses an array of x-ray detectors to detect an attenuated x-ray beam that has passed through the body of a human or animal subject. The detector data are processed by a computer system to generate image data representing a recognizable view of the interior structure of the imaging object.
CT techniques are valuable in a wide range of application areas where noninvasive and nondestructive examination of internal structures is needed. Medical applications include imaging of emissions from radioactive substances introduced into the subject (single photon emission CT, positron emission CT, etc.), as well as x-ray transmission CT. Non-medical applications include, for example, non-destructive testing and inspection, mineral deposit mapping (microseismic CT imaging), and three-dimensional image generation in electron microscopy.
A CT system generates a display image from data representing measurements of energetic signals transmitted through or emitted from a subject in a range of directions. This process is "tomographic" in that structural details of a subject are represented as a cross-sectional view along a given plane through the subject. The process is "computerized" because the raw detector data only indirectly represent a view of the subject. Substantial data processing is required to convert the raw data into a recognizable view of the internal features of the subject.
Computer processing of CT detector data is necessary because the data correspond to mere projections of the subject structure along various different paths. The differences between the data along different paths, in relation to the spatial separation of the paths, provide an indirect representation of the interior structure of the subject. However, to obtain a recognizable view of that structure requires processing of the projection data from the detectors by a so-called reconstruction algorithm.
The image data representing a CT image are therefore the product of considerable computer processing applied to projection data collected by the detector array. This indirect relationship between image data and detector data distinguishes CT imaging from direct photographic imaging technologies. For example, a charge-coupled device (CCD) camera comprises an array of solid state photosites each detecting incident radiation (such as visible light) at a corresponding position. Such a camera produces an image in which each picture element ("pixel") directly corresponds to a photosite in the CCD array.
In contrast, a CT system produces images in which each pixel is reconstructed from data generated by many detectors. The image data representing the image therefore do not correspond directly to individual detectors in the detector array. Instead, each image pixel will typically include contributions from the projection data of all the detectors in the system.
The indirect relationship between image data and detector data seriously complicates the problem of recognizing the presence of a faulty detector in a CT system. The presence of a bad photosite in a CCD camera will generally be apparent from localized artifacts in the CCD image. In a CT image, however, the relationship between a bad detector and an image artifact caused by the bad detector is less direct. This is because the CT image has been generated through a reconstruction algorithm, which has combined the bad detector data together with the data from the other detectors in the detector array.
In current CT systems, constructed according to the existing third-generation architecture (to be described below), the existence of a bad detector shows up in the CT image as a so-called "ring" artifact. Trained CT technicians can readily identify such ring artifacts in a generated CT image. The location of the ring in the image will also typically provide some information about the location of the bad detector in the detector array.
Reliance on ad hoc visual inspection, however, leaves much to be desired for reliable and repeatable identification of bad CT detectors. Fourth generation systems currently under development are expected to avoid generating the ring artifacts upon which such inspection depends. Even for current systems, the reliability of visual inspection depends on the care and accuity with which individual CT images are inspected.
Moreover, current CT systems typically implement post-processing techniques to remove ring artifacts. These techniques reduce the local image distortion caused by the ring, but they also inevitably cause a loss of some relevant image information. The local image distortion of the ring artifact is also the image feature that makes the existence of a bad detector discernable by visual inspection of the image. Thus, post-processing to clean up CT images has the unfortunate side effect of making the existence of a bad detector even more difficult to identify.
In view of these problems, users of CT systems need a practical system and method for detecting the existence and location of a bad detector in a CT detector array. Such a system should provide reliable, repeatable monitoring of the detector data to determine whether the data contains a signature indicative of a bad detector. Preferably, such a system would perform its monitoring function in a background mode transparent to the normal operation of the CT system. Ideally, the monitoring system would alert a responsible party to the likely presence of a bad detector and would also estimate the position of the bad detector in the detector array.